Continuous casting process



Nov. 28, 1967 o. B. ATKIN 3,354,936

CONTINUOUS CASTING PROCESS Filed May 26, 1965 INVENTOR. OLIVER B. ATKINATTORNEYS United States Patent 3,354,936 -CONTINUOUS CASTHNG PROCESSOliver B. Atkin, Harnden, Conn., assignor to Anaconda American BrassCompany, a corporation of Connecticut Filed May 26, 1965, Ser. No.459,040 2 Claims. (Cl. M t-82) ABSTRACT OF THE DISCLOSURE A continuouscasting process for preventing harmful surface deposits on castingscommonly produced during the continuous casting of copper metal alloyshaving as one element a metal of a higher vapor pressure than copperwherein the metal having the high vapor pressure distills from thesurface of the hot metal shape after solidification and condenses on thesurface of the mold. In this process the metal shape is withdrawn in afirst direction toward the exit end of the mold and is thencylindrically stopped and reversed to reabsorb the condensed metal onthe reversed portion of the metal shape which is brought intojuxtaposition with the condensed metal.

This invention relates to a continuous casting process, and moreparticularly, it relates to the prevention of harmful surface depositson castings produced by the continuous casting of metal alloys having as'one element a metal of relatively high vapor pressure which issegregated from the metal during casting and periodically reabsorbed onthe surface of the metal shape.

In the continuous casting of metal alloys, particularly copper alloys,there occurs a troublesome segregation of certain metals from thesurface of the casting which become deposited on the mold wall and arereabsorbed on the surface of the metal in a discontinuous pattern. Thissurface discontinuity is harmful in that it forms brittle spots on thecasting. One theory on thernechanism by which the segregation of metaland subsequent reabsorption on the surface of the cast metal occurs isdescribed in Metals Technology, volume 8, No. 2, in an article by DanielR. Hull entitled, Some Practical Observations on Inverse Segregations. v

Attempts have heretofore been made in an effort to eliminate the surfacediscontinuities; one such process is described in US Patent No.2,740,177 and relates to intermittent withdrawal of a metal casting forthe purpose of maintaining molten metal stationary with respect to amold in a chill zone until the outer surface of the metal is frozen intoa shell to prevent bleeding of metal constituents to the surface of thecasting and freezing on the outer surface shell. I have found that thismethod-does not completely solve certain surface discontinuities on thecasting.

The operation of continuously casting metal shapes is initiated bypouring molten metal into a crucible of an assembly for maintaining themetal in its molten state. The crucible has an outlet, usually in thebottom, and has a mold in which the molten metal is shaped andsolidified connected to the crucible outlet. The mold is usually open atboth of its ends, and as the molten metal solidifies into the metalshape defined by the mold, it contracts away from the walls of the moldand, therefore, can be linearly withdrawn from the mold on a continuousbasis.

Because the molten metal contracts away from the walls of the mold assoon as it solidifies, a separation is formed between the walls of themold and the metal shape. I have discovered that in the continuouscasting of copper alloys for example, after the liquid metal hassolidified in the mold during continuous casting, certain alloyingelements, for example zinc, tin and lead, distill from the surface ofthe hot solidified casting because of their relatively high vaporpressure, and this metallic vapor condenses on the surface of the coldmold in the separation formed by the contraction of the metal shape.This condensed metal builds up to the point where it touches thesolidified casting where it is reabsorbed as a ring of zincrich,tin-rich or lead-rich material on the surface of the casting. In oneexample, when casting with a Cu30% Zn alldy, localized ring portionsformed by reabsorption of segregated zin contained 30% Cu and 70% Zn;these local portions were very low in hot strength and ductility and thesurface was easily broken along these portions and prolonged casting,say 20 minutes, will cause rods to break apart along the portions whichreabsorbed the zinc.

I have found that by stopping and reversing the withdrawal of thecasting periodicaly for a fraction of the time the casting is withdrawn,the build-up of condensed metallic vapor on the mold in the separationbetween the metal shape and the mold is reabsorbed by the casting longbefore it can accumulate and become harmful.

Broadly stated, the invention is a method of continuous casting metalshapes wherein molten metal is fed through a chilled mold, solidifiedwithin the'rnold and contracted away from the walls of the mold whilebeing continuously fed therethrough. The improvement is in castingalloys which are alloyed with metals of differing vapor pressures sothat the metal having the higher vapor pressure distills from thesurface of the hot metal shape after solidification and condenses on thesurface of the mold within the separation. The process is characterizedby withdrawing the metal shape in a first direction toward the exit endof the mold and cyclicaly stopping and reversing the casting directionof the metal shape for a fraction of the time period of withdrawal,reabsorbing the condensed metal onto the reverse-d portion of the metalshape which is brought into juxtaposition with the condensed metal, andwithdrawing the metal shape in the first direction again.

A preferred embodiment of the invention is described hereinbelow withreference'to the drawing wherein:

FIG. 1 is aside elevation partly in section if continuous castingapparatus; and I v FIG. 2 is an enlarged fragmentary side elevation ofapparatus for withdrawing the metal shape being cast and cyclicallyreversing the direction of withdrawal.

The continuous casting apparatus shown in the drawing consists of acrucible 10 which is shown substantially filled with molten metal 11.The crucible is mounted within a holding furnace assembly 12 which isconstructed of an outer metal sheet 13 and is lined with a heatinsulating material 14. A plurality of burners 15 extend through theside walls of the holding furnace 12 at spaced intervals to supply thenecessary heat to the crucible to keep the metal molten. I

At the bottom of the crucible 10 is a crucible outlet 16 which leadsinto an elongated annular furnace opening 17. The annular openingextends through the insulating 3 material 14 and the outer sheet 13 atthe bottom of the holding furnace assembly 12.

A cooling zone 18 below the crucible outlet is defined in part by acopper cooling block 19 which is supported on a plate 20, and the platein turn is supported on a pair of threaded bolts 21 which depend fromthe bottom of the holding furnace assembly 12. The cooling block has aplurality of water passages 22 formed therein so that a coolant such ascold water can be circulated through the block to maintain it cooledduring operation. An annular cooling block opening 23 extend axially andcompletely through the cooling block in co-axial alignment with thecrucible outlet 16 and the annular furnace opening 17.

Positioned with its marginal inlet end 24 within the crucible outlet andextending through the annular furnace opening 17 and through the annularcooling block opening 23 and in co-axial alignment with the crucibleoutlet is a graphite mold 25. The mold is held in place in the crucibleoutlet 16 and the annular furnace opening 17 by a deposit of refractorycement 26. As shown, the inlet end 24 of the mold extends above thebottom of the crucible slightly for the reason that this extension willinsure that the metal flowing into the mold will not be partially cooledby the crucible bottom; this is particularly important at the start. Anintermediate portion 27 of the graphite mold is within the cooling zoneand its marginal outlet end 28 extends outwardly from the bottom of thecooling block. The cooling block 19 surrounds a major length of theintermediate portion 27 of the graphite mold and is in heat exchangerelationship therewith. At the marginal outlet end 28 of the mold is anoutwardly flared portion 29 which seats in an annular recess 30 providedin the cooling block to retain the mold in the block againstlongitudinal displacement in the direction of travel of the metal shapepassing therethrough. Spaced from the outlet end of the graphite moldare a set of rollers 31 which are powered to withdraw the metal shapefrom the mold.

In FIG. 2 the rollers 31 are shown connected to a common geararrangement 32 for rotating the rollers together at the same speed andone of the gears is connected to a shaft 33 which is coupled with areversible motor 34 for driving the rolls in either direction. Motor 34can have a suitable gear reducer attached to it so that the rollers canbe driven in either direction at the relatively slow speeds required forwithdrawal of the metal shape.

In continuous casting, the molten metal is maintained in crucible andthe metal is gravity fed through the crucible outlet 16 into the mold 25where it is cooled and solidified. The mold is kept cold by the coolingblock, and as the metal solidifies it solidifies first on the outermostsurface and continues to solidify progressively inward. Upon initialsolidification the metal shape will contract away from the walls of themold leaving a separation between the cast metal shape and the moldwalls. It is theorized that upon initially breaking contact with thecool walls of the mold, there is a tendency for the metal shaperepeatedly to expand in contact or near contact with the cooled walls ofthe mold again, at which position the metal shape contracts once again.There is a point at which substantial solidification has occurredleaving a separation between the metal shape and the walls of the mold.

After solidification, when castipg with an alloy having at least oneelement with a relatively high vapor pressure, the metal distills outfrom the surface of the metal shape and the metal vapor condenses on thecold walls of the mold where it accumulates until reabsorbed on thesurface of metal shape, usually in the form of a brittle surface defect.Examples of the alloys which experience this segregation of a metalduring casting are copper alloys which are rich in zinc, tin, lead, oreven sulfur or tellurium.

According to my method, the metal shape is withdrawn in a firstdirection toward the exit end of the mold in its normal castingdirection for a time period which, dependent upon the alloy being cast,causes some amount of condensation of metallic vapor on the mold wallwithin the separation but not such an accumulation as will be harmful tothe metal casting if reabsorbed on its surface. This time period must bedetermined empirically, based upon each alloy being cast and the degreeof vapor distillation which takes place for a given length of time. Theessential feature of the method is, however, that the metal shape isstopped and simultaneously reversed so that a portion of the freshlycast metal shape is brought in juxtaposition with the condensed metal onthe mold wall and reabsorption of the condensed metal on the reversedportion on the metal shape takes place. It is theorized that uponreversal of the metal shape the reversed surface portion will contactthe condensed metal and thereby become reabsorbed. This theory is basedupon the assumption that the reversed portion is expanded to a slightlylarger diameter and upon reversal will contact the condensed metal. Bycyclically stopping and reversing the casting direction of the metalshape for a small fraction of the time period of withdrawal, the amountof reabsorbed metal is not harmful to the metal shape cast. The reversalis easily accomplished by an arrangement of standard parts as shown inFIG. 2.

The mold is preferably made of a graphite which has been treated torender it substantially non-porous. Other condense materials can be usedprovided the distilled metal cannot penetrate the material and they havegood thermal conductivity, good rigidity and low thermal expansion.These graphite molds are also characterized by the property that theyare not wet by copper alloys and therefore are particularly preferredwhen this method is used for the casting of copper alloys. It is thoughtthat if the mold can be wet by the alloy (e.g. a metal mold) thisperiodic reversal of the metal shape will tend to impart physicaldefects on the surface of the metal shape during reversal.

In one example, a copper alloy containing 87% copper and 13% zinc wascast at a rate of 13" per minute. It was withdrawn in the castingdirection for a time period of 3 seconds, and it was stopped andreversed for a time period of 1 second. This cyclical stopping andreversing effected about A reversal of the metal casting and this wassufiicient to reabsorb the condensed Zone from the cold mold wall.

In another example a copper alloy containing 90% copper and 10% tin wascast at a rate of 15" per minute. With this alloy a 20-second timeperiod in the casting direction and a 1 second stopping and reversalperiod was sufiicient to absorb the condensed tin before any harmfulaccumulations took place.

In a third example a copper alloy containing about 99% copper, 1% leadand .10% boron was cast at a rate of 15" per minute with a time periodof 10 seconds in the direction of casting and 1 second for stopping andreversal. This effectively removed the condensed lead from the moldwalls and reabsorbed it on the surface without causing harmful surfacedefects.

I claim:

1. In a method of continuously casting metal shapes wherein molten metalis fed through a chilled mold, solidified within the mold and contractedaway from the walls of the mold while being continuously fedtherethrough, the improvement comprising casting copper metal alloyswhich are alloyed with metals having a higher vapor pressure than copperso that the metal having the higher vapor pressure distills from thesurface of the hot metal shape after solidification and condenses on thesurface of the mold within the separation formed by said contraction,said mold surface characterized by not said high vapor pressure metalbeing selected from the being wet by the alloy being cast, withdrawingthe metal group consisting of lead, tin or zinc.

shape in a first direction toward the exit end of the mold,

cyclically stopping and reversing the casting direction of ReferencesCited the metal shape for a fraction of the time period of with- 5UNITED STATES PATENTS drawal, reabsorbing the condensed metal onto there- 2,740,177 4/1956 Smart versed portion of the metal Shape Which isbrought into 3 290 734 12 19 Wertli 1 4 2gz juxtaposition with thecondensed metal, and with drawing the metal shape in said firstdirection again. SPENCER OVERHOLSER Pnmary 2. A method according toclaim 1 characterized by 10 R. D. BALDWIN, Assistant Examiner.

1. IN A METHOD OF CONTINUOUSLY CASTING METAL SHAPES WHEREIN MOLTEN METALIS FED THROUGH A CHILLED MOLD, SOLIDIFIED WITHIN THE MOLD AND CONTRACTEDAWAY FROM THE WALLS OF THE MOLD WHILE BEING CONTINOUSLY FEDTHERETHROUGH, THE IMPROVEMENT COMPRISING CASTING COPPER METAL ALLOYSWHICH ARE ALLOYED WITH METALS HAVING A HIGHER VAPOR PRESSURE THAN COPPERSO THAT THE METAL HAVING THE HIGHER VAPOR PRESSURE DISTILLS FROM THESURFACE OF THE HOT METAL SHAPE AFTER SOLIDIFICATION AND CONDENSES ON THESURFACE OF THE MOLD WITHIN THE SEPARATION FORMED BY SAID CONTRACTION,SAID MOLD SURFACE CHARACTERIZED BY NOT BEING WET BY THE ALLOY BEINGCAST, WITHDRAWING THE METAL